Here’s a little and pretty straight forward example of what I am talking about.

First a little disclaimer: This is not rocket science and I am sure there are other ways to do this (Plug-Ins, even) and there are plenty of reasons why you don’t want to add fake stuff like this, but it’s just another tool at your disposal. Who knows what client/project you might need it for?

So let’s take this still from the short film ARPEGGIO. Let’s assume for some contrived reason, that we want to give the practical lamp behind the actress some halation, spilling into the foreground.

For starters, I draw a mask around the practical, roughly matching its shape. If the camera would be in motion, I’d have to track the mask too, as well as compensate for possible occlusions, in this case, if the actress were to more in front of the light.

Alright then: I add a real big outside falloff and shape it to my liking. Here, I want the face to be a bit more affected than her arm. Unfortunately, the falloff is always linear in Resolve, it would be really nice choosing between different interpolations like quadratic, cubic, exponential, etc. like in Nuke. For that kind, I’d have to “stack” masks with linear falloff, but that’s a whole different subject.

And here comes the “trick”: I now lift just the blacks and shift them towards the color of the light.

I don’t touch neither gamma nor highlights, so the light won’t be prone to clipping or other kinds of “unnatural” behavior. Well, you could also lift and tint the gamma sparingly, I guess…

The thing you have to mind is that noise and artifacts in your blacks will also get amplified, so you either want to de-noise your footage first, add some post-grain after the effect, and/or lift the blacks really cautiously.

So that’s all, folks. I hope, any of you found this useful!

Before:

And after:

EDIT 2017-03-01: Jesse Glucksman, C.S.I., points out that a similar kind of softening can be achieved by a luma key and locally decreasing the Midtone Detail (MD). Thanks, Jesse!